System for taking displacement measurements having photosensors with imaged pattern arrangement

ABSTRACT

The present invention is generally directed to a system for taking displacement measurements of an object. The invention utilizes the Moiré effect to take precise displacement measurements of an object. In this regard, a visible pattern is disposed on an object, and a plurality of photosensors are uniformly spaced apart from the visible pattern. Importantly, the spacing between the photosensors is slightly different than the spacing between light and dark lines forming a projection or image of the visible pattern. This allows the invention to utilize the Moiré effect to accurately compute precise displacements or movements of the object. In this respect, electrical signal generated by the photosensor array will embody a repeating envelope pattern resulting from the difference in the pitch of the photosensors and the pitch of the projection or image of the visible pattern. This envelope has a spatial frequency that is significantly lower than the fundamental repetition frequency of either the image (or projection) of the visible pattern or the photosensor array, where these frequencies of the image or projection of the visible pattern and the photosensor array are equal to the reciprocal of the distances separating adjacent repetitions of pattern within the image of the visible pattern or the reciprocol of the pitch of adjacent photosensor elements respectively. Thus, small lateral motion of the object bearing the visible pattern, made parallel to the direction of the repetition of the repeating patterns, produces a relatively large shift in the position of the signal envelope which has a lower spatial frequency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to measurement systems, and moreparticularly to a system for taking displacement measurements of anobject having a regular pattern preprinted thereon.

2. Discussion of the Related Art

There are a wide variety of known systems in which detecting and/ormeasuring the position or displacement of an object is important. Forexample, the system disclosed in U.S. Pat. No. 5,578,813, assigned tothe assignee of the present invention and also co-invented by theinventor of the present invention, is a system and method fordetermining relative movement between a hand-held scanner and a web ofmaterial (i.e., piece of paper). Specifically, that system utilized anillumination/imaging sensor to detect relative movement between thescanner and the web of material by identifying structure-relatedproperties of the web of material. The inherent structure-relatedproperties of the web (such as paper fibers, or other constituents) wereused for navigational purposes, namely to identify the navigational pathof the scanner, so that the image scanned could be reconstructedelectronically.

Another system is disclosed in U.S. Pat. No. 5,291,131, which disclosesan apparatus for measuring the elongation of a circulating chain(elongation resulting from component wear, stretching, or otherwise).The system disclosed therein uses two sensors (e.g., magnetic oroptical) disposed a predetermined distance apart along the path of thecirculating chain. The distance between the two indices is calculated onthe basis of the calculated speed of the moving body and the time whichelapses from when a first index passed a first sensor until a secondindex passed a second sensor. By continuing this observation over time,and comparing the calculated distances, chain elongation can bemeasured. Further, similar systems predating the '131 patent includethose disclosed in U.S. Pat. Nos. 4,198,758 and 4,274,783, both entitled“Chain Measuring and Conveyor Control System”, co-invented by theinventor of the present invention.

As is also known in the prior art, the print head of some ink jetprinters is configured to move across the print width of a sheet ofpaper. The deposition of ink from the print head to the sheet of paperis closely controlled based in part on positional information of theprint head. In some such printers, this positional information isobtained by stretching a transparent sheet of material across the spancovered by the width of the sheet of paper. This sheet of material ispassed through a slot in the print head, and containsperiodically-spaced demarcation lines. An optical emitter/detector pairis disposed across the slot, and is configured to count the demarcationlines. By maintaining an accurate count of the demarcation lines, thesystem can maintain information related to the position of the printhead along the sheet of paper. In such a system, the sheet of materialis held stationary, while the emitter/detector pair (which is fixed tothe print head) moves in relation to the sheet of material.

In systems like those mentioned above, as well as many other systems,there is a desire to obtain accurate and precise displacementmeasurements of an object. Accordingly, it is desired to provide anapparatus and method that effectively measures object displacements.

SUMMARY OF THE INVENTION

Certain objects, advantages and novel features of the invention will beset forth in part in the description that follows and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned with the practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the devices and combinations particularly pointed out in theappended claims.

To achieve the advantages and novel features, the present invention isgenerally directed to a system for taking displacement measurements ofan object. In accordance with one aspect of the invention, the systemincludes an object having a visible pattern disposed thereon (the term“visible pattern” will be used throughout to represent any patternsensed with a photosensor array, whether or not the illuminationinvolved is in the visible spectrum or not). In this regard, the patternis defined by areas of systematically alternating contrast (e.g. blackand white or other colors). A plurality of photosensors are uniformlyspaced apart from the visible pattern, and are further disposed in aconfiguration similar to the visible pattern. In this regard, if thevisible pattern is a repeating circular pattern, then the photosensorsare disposed in a repeating circular pattern. If the visible pattern isa plurality of parallel, linear demarcations, then the plurality ofphotosensors may be disposed in a linear array. A lens is disposed toimage the visible pattern onto the plurality of photosensors.Preferably, the pitch, or spacing between adjacent photosensor elements,is slightly different from the spacing in the image of the contrastingareas defining the visible pattern. As will be discussed in furtherdetail below, this allows the invention to utilize the Moiré effect totake precise displacement measurements of the object.

In accordance with this aspect of the invention, a circuit iselectrically connected to the plurality of photosensors, wherein thecircuit is configured to evaluate an electrical signal generated by theplurality of photosensors to determine the displacement of the object.In this regard, the electrical signal will embody a repeating envelopepattern resulting from the difference in the pitch of the photosensorsand the pitch of the image of the visible pattern. This envelope has aspatial frequency that is significantly lower than the frequency ofeither the visible pattern in the image or the photosensor array, wherethe frequencies of the visible pattern in the image and the photosensorarray are equal to the reciprocals of the distances separating adjacentpattern demarcations in the image or adjacent photosensor elementsrespectively. In this regard, lateral motions of the object bearing thevisible pattern, made parallel to the direction of the repetition of therepeating patterns, produces a shift in the position of the lowerspatial frequency signal envelope. Even slight displacements of theobject bearing the visible pattern can be readily detected since theycause relatively large displacements of the envelope pattern whichitself has a lower spatial frequency. Object motions that includerotation within the object plane of the imaging optics produce morecomplicated motions of the envelope signal but also allow increasedprecision in detection of these generalized motions.

In accordance with this preferred embodiment, the lens is a telecentriclens. A telecentric lens may include an aperture at a focal distancebehind a first lens or lens group to prevent changes in an object'sfield position from causing magnification changes. A second lens may beplaced behind the aperture at a distance equal to its focal length, forthe purpose of similarly preventing changes in the focal position of adetector from also causing magnification changes. The individual lensesof the telecentric lens are aligned along a central axis.

Depending upon system application needs, another but alternativeimplementation doesn't require an imaging lens. In this form ofimplementation, an object surface may be placed in close proximity tothe photosensor array with illumination provided through the object orfrom light sources interstitial to the array.

In accordance with another aspect of the invention, a system is providedfor measuring the tilt of an object and doesn't require use of patternsthat are necessarily systematic. In this aspect, a first photosensorarray is disposed alongside a central axis and angled with respect tothe central axis. Similarly, a second photosensor array is disposedalongside the central axis and, like the first photosensor array, isangled with respect to the central axis. Indeed, the second photosensorarray is angled at an angle substantially opposite the angle of thefirst photosensor array. Further, the first and second photosensorarrays are disposed on a side of the telecentric lens opposite theobject. Finally, a circuit is provided in electrical communication withthe first and second photosensor arrays. The circuit is configured toevaluate electrical signals output from the first and second photosensorarrays to determine the tilt of the object.

In one embodiment, the photosensor arrays may be one dimensional arrays(e.g., either a column or a row of photosensor elements). In such anembodiment, the system will detect tilt in one dimension of the object(e.g., left-right tilt). In an alternative embodiment, the photosensorarrays may be two dimensional arrays. In such an embodiment, the systemwill detect tilt in either (or both) of two dimensions. Alternatively, abeam splitter may be used to allow the pair of tilted arrays to beoptically centered on a central axis without physical interference.

In accordance with yet another embodiment of the invention, anothersystem is provided for tracking the movement of a target object inthree-dimensional space. In accordance with this aspect of theinvention, the system includes two different lenses oriented in asimilar, but slightly different direction. More particularly, a firstlens having a relatively deep depth of field is disposed along a firstoptical axis. Similarly, a second lens also having a relatively deepdepth of field is disposed along a second optical axis. The secondoptical axis is slightly angled with respect to the first optical axisso that they are not parallel, but in such a way that the first andsecond lenses have a shared field of view in which a target object, fortracking, may be located. In this regard, a first photosensor array isdisposed substantially orthogonal to the first optical axis and oppositethe shared field of view. Likewise, a second photosensor array isdisposed substantially orthogonal to the second optical axis andopposite the shared field of view. Finally, a circuit is disposed inelectrical communication with the first and second photosensor arrays,wherein the circuit is configured to evaluate electrical signals outputfrom the first and second photosensor arrays to track the movement of anobject within the shared field of view. The evaluation of the outputsfrom the photosensor arrays may involve correlation between the twoimages or may use more heuristic algorithms. As discussed above,contrast patterns may furthermore be manifested on the object to enableMoire amplification of lateral displacements to achieve higher precisionif telecentric lenses are used or if object displacement componentsparallel to the optical axis are small and magnification changes aretracked.

As is known by persons in the art, a variety of systems use correlationtechniques of various types to measure image displacements betweensequential images of a moving object. For example, U.S. Pat. No.5,729,008 entitled “Method and Device for Tracking Relative Movement byCorrelating Signals from an Array of Photoelements,” U.S. Pat. No.5,703,353 entitled “Offset Removal and Spatial Frequency Band FilteringCircuitry for Photoreceiver Signals,” U.S. Pat. No. 5,686,720 entitled“Method and Device for Achieving High Contrast Surface Illumination”,U.S. Pat. No. 5,644,139 entitled “Navigation Technique for DetectingMovement of Navigation Sensors Relative to an Object”, all assigned tothe assignee of the present invention, and U.S. Pat. No. 5,578,813referenced above all discuss various correlation techniques, and areeach incorporated herein by reference. Such techniques may be employedby the novel structures of the present invention, and need not bedescribed herein.

DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated in and forming a part of thespecification, illustrate several aspects of the present invention, andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIGS. 1A-1C are drawings that illustrate the Moiré effect as two sets ofidentically-sized concentric circles are moved laterally with respect toeach other;

FIGS. 2A-2B are drawings that illustrate the Moiré effect as two sets ofparallel lines are rotated with respect to each other;

FIGS. 3A-3B are drawings that illustrate the Moiré effect as two sets ofidentically-sized concentric triangles are moved laterally with respectto each other;

FIG. 4A is a block diagram illustrating one embodiment of the presentinvention that utilizes the Moiré effect of a relatively simple patternof horizontal lines to measure the displacement of an object bearing thepreprinted pattern;

FIG. 4B is a block diagram similar to that illustrated in FIG. 4A, butillustrating a pattern of skewed lines;

FIGS. 5A and 5B are diagrams of a system used to measure the tilt of anobject, constructed in accordance with an alternative embodiment of theinvention;

FIG. 6A is a diagram illustrating a hypothetical light pattern sensed bytilted one-dimensional photosensor arrays for an untilted objectpositioned at a central focal plane;

FIG. 6B is a diagram illustrating a hypothetical light pattern sensed bytilted one-dimensional photosensor arrays for an untilted objectpositioned out of a central focal plane;

FIG. 6C is a diagram illustrating a hypothetical light pattern sensed bytilted one-dimensional photosensor arrays for an object that is tiltedabout an axis that is parallel to the axis of tilt of the photosensorarrays, lies in the central focal plane and intersects the optical axis;

FIG. 7 is a diagram illustrating a hypothetical light pattern sensed bytilted two-dimensional photosensor arrays for an object that is tiltedabout an axis that is perpendicular to the axis of tilt of thephotosensor arrays, lies in the central focal plane and intersects theoptical axis; and

FIG. 8 is diagram illustrating an alternative embodiment of the presentinvention that may be utilized in a three-dimensional tracking system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Having summarized various aspects of the present invention, referencewill now be made in detail to the description of the invention asillustrated in the drawings. While the invention will be described inconnection with these drawings, there is no intent to limit it to theembodiment or embodiments disclosed therein. On the contrary, the intentis to cover all alternatives, modifications and equivalents includedwithin the spirit and scope of the invention as defined by the appendedclaims.

Reference is now made to FIGS. 1 through 3, which illustrate a few verysimple Moiré patterns, or patterns illustrating the Moiré effect.Broadly stated, the Moiré effect is an optical phenomenon resulting fromthe superposition of two (or more) similar periodic or quasi-periodicstructures. For example, when two line/space grids—such a grid beingconsidered as any one or two dimensional pattern of alternating dark andlight (or color varied) bands (herein loosely called lines) of aregularly repetitive nature—are superimposed, the intersections of thelines (or spaces) of the two grids determine another repetitive patterncalled a Moiré pattern. Although Moiré patterns and the Moiré effecthave been well documented and are understood by persons skilled in theart, a few simple patterns have been depicted herein for completenessand illustration.

In this regard, FIGS. 1A through 1C illustrate the Moire effect using asimple pattern of bands in concentric circles. Specifically, thedrawings illustrate a pattern of duplicate concentric circles disposedto overlap in varying degree. FIG. 1A depicts that concentric circlessubstantially overlapping, while FIG. 1B illustrates a greaterseparation of the centers of the patterns, and FIG. 1C illustrates aneven further separation of the two patterns. As shown, each of thedrawings is characterized by a unique overlap between the two patterns,which provides different visual effects. It can be noted in these casesthat the patterns which develop are curved lines that are hyperbolic inshape. For example, dashed lines are provided in FIG. 1A to illustratethe pattern. There is illustrated a central, substantially vertical,dashed line and two horizontally offset curved (hyperbolic) dashed linesthat follow a pattern formed by the overlapping bands. Although notillustrated with further dashed lines, it can be observed that as theconcentric patterns move further apart (FIG. 1B) more patterns result,and the curves increase in radius.

FIGS. 2A and 2B illustrate a Moiré effect that results when two sets ofequally-spaced parallel lines or bands are rotated with respect to oneanother. As illustrated in these drawings, when oriented at acute anglesto one another, the lines form gratings (parallelograms), wherein thelength of the parallelograms increases as the two patterns of lines arerotated into closer alignment. Note how visually tracing linesvertically through these intersections in FIG. 2A produce a pattern ofvertical lines spaced apart horizontally by a relatively large distance.Wherein similar lines traced (mentally) in FIG. 2B become tilted fromvertical and more closely spaced apart.

FIGS. 3A and 3B illustrate yet another pattern of lines. In thisfigures, an identical set of concentric triangles are shown. In FIG. 3Athe two sets of triangles are lightly offset (horizontally), while inFIG. 3B is offset is larger. Again, the movement of one set of the lineswith respect to the other set creates a certain visual effect withinregions where lines from the two patterns overlap (only partiallydetectable from the two figures). Numerous publications related to Moirépatterns have presented mathematical explanations for the patterns.However, as will be appreciated from the discussion herein, the presentinvention is not limited to any given set of Moiré patterns, andtherefore it is unnecessary to present an extensive mathematicaldiscussion herein. Suffice it to say, that one of ordinary skill in theart will have a sufficient understanding of the relevant mathematics toimplement the concepts and teachings of the present invention.

In addition to those described above, other types of Moiré patterns andMoiré effects are known. For example, a different Moiré effect occurswhen two set of equally-spaced parallel lines are overlayed.Specifically, when the spacing separating the lines of the first set isslightly different from the spacing separating the lines of the secondset, a different effect is noted when the lines are moved laterally withrespect to one another—i.e., in a direction orthogonal to these lines.The visual effect resulting from this movement will be different fromthe visual pattern that would result if both set of lines had the samespacing. In fact, small differences in pitch effect an interactionpattern of lower spatial frequency than the individual pattern-pitchfrequencies, but more importantly, these interaction patterns move morethan the displacement of the two separate patterns.

In this regard, reference is now made to FIG. 4A, which illustrates afirst embodiment of a system 10 for taking displacement measurements,constructed in accordance with the present invention. This system 10utilizes the Moiré effect in a unique way, in order to carry-outdetailed measurement calculations. Central to the invention is aphotosensor array 12, which contains a plurality of photosensorelements. The photosensor elements are spaced apart by a regular anduniform distance, creating a natural pitch for a pattern of photosensorelements.

An object 14, includes a preprinted visible pattern 16. The pattern is asystematic, regular pattern which, for example, may be printed on thesurface of the object. For purposes of the invention, it does not matterwhether the object is a substantially two dimensional object, such as apiece of paper, or whether it is a three dimensional object, having thepattern 16 printed on one side thereof. Indeed, the broad concepts andteachings of the present invention are applicable to various fields andenvironments. What is significant for purposes of the present inventionis that the systematic pattern of the pre-printed pattern, as imaged orotherwise projected onto the photosensor array, bear a close relation tothe spacing of the photosensor elements comprising that array. In thisregard, the pattern of FIG. 4A, as imaged or otherwise projected ontothe photosensor array, is a series of equi-distant parallel thick linesthat are spaced apart by a distance that is slightly different from thespacing between every other of the individual columns (or rows) of thephotosensor elements of the photosensor array.

A lens 18 is disposed between the photosensor array 12 and the object14, to project the preprinted pattern 16 onto the photosensor array.Note that the layout geometry of the elements comprising the photosensorarray is what forms a second pattern which, along with the projectedimage of the preprinted pattern on the object, creates a Moiré effectwithin the outputs of the photosensor array. The photosensor elementscan be made of CCDs, CMOS devices or amorphous silicon devices. Thephotosensor array 12, in turn, is electrically connected to a circuit20, which is configured to evaluate the outputs of the plurality ofphotosensor elements. Although the system 10 need not graph theelectrical signals received from the photosensor array 12, a graph 30 isprovided in FIG. 4A to depict the signal values received by the circuit20 from the photosensor array 12. In this regard, a plurality ofdiscrete signal lines 31, 32, 33 are shown, each being defined by anamplitude. The individual signal amplitudes correspond to the amplitudesof the electrical signals output from the photosensor elements. In thisregard, the photosensor elements output an electric signal thatcorresponds to light originating from, transmitted through, reflectedfrom, refracted by, or diffracted by the surface containing the pattern.The graph 30 is illustrative of the signal values of one row of thephotosensor array 12.

The pattern 16 illustrated in FIG. 4A consists of a series ofequally-spaced parallel and thick lines, preferably with equal mark andspace distances. As the image or projection of one of these lines alignswith a photosensor element, the magnitude of the electrical signaloutput from that photosensor element is relatively low. As the object 14begins to move and the image or projection of the line begins to shiftfrom its alignment with the photosensor element, then the magnitude ofthe electrical signal from the photosensor element begins to increase.The magnitude of the electrical signal reaches a maximum when thepattern line is completely out of phase with the photosensor element.Thus, when the image or projection of the patterned lines are spacedapart by a slightly different spacing from that which separates thephotosensor elements, the signal magnitudes output from one row ofphotosensor elements may be as illustrated in the graph 30.Specifically, although the photosensor elements and the image orprojection of the patterned lines may be spaced relatively closelytogether, the graph defines a signal envelope having a much lowerspatial frequency.

The circuit denoted by reference numeral 20 includes, at least in part,a processing circuit 22 that is configured to evaluate the signalsoutput from the photosensor array 12. In this regard, the circuit of thepreferred embodiment of the invention may include a central processingunit (CPU) 24, a memory 26, and some program code for executing on theCPU 24. In this regard, the program code is stored within the memory 26,and it embodies software that is configured to control the operation ofthe CPU 24 to perform the calculations involved in the evaluationperformed by the system 10. Although the software may be embodied indifferent forms, its will preferably include a segment 28 that evaluatesthe phase of the envelope 34 of the signals generated from photosensorarray 12.

Thus, from the system described above, a very slight movement of theobject 14 effects the phase of the lower spatial frequency or envelope34 is a readily quantifiable way. That is, a very slight movement of theobject 14 carrying the preprinted pattern 16, results in a much largerchange in the envelope 34 of the signal 30.

The Moire patterns that can result from rotational misalignment betweenthe pattern 16 and the photosensor array 12 can be detected andaccommodated algorithmically to produce correct measurements of bothdisplacement and rotation. Preferably, the lens (i.e., . . . ) 18 willbe one or two lens groups and an optical stop, forming a telecentriclens. As will be appreciated, the use of a telecentric lens 18 allowsthe pattern surface to deviate from the focal plane of the lens, withoutsignificantly affecting the pitch of the image pattern at the array, byan otherwise change in effective magnification.

Although illustrated as a two dimensional array of photosensor elements,it will be appreciated that the photosensor array 12 may be implementedas a one-dimensional array of photosensor elements. If the onlymeasurement of interest is a one-dimensional movement (e.g., left/rightmovement) of the object 14 and rotational alignment is not an issue,then a one dimensional array or photosensor elements is all that isrequired. However, a two-dimensional photosensor array 12 (asillustrated) is desired, as it is capable of measuring both left/rightmovement of the object, both up/down movement, and rotational movementof the object.

As is illustrated by the waveforms 30 and 130 in FIGS. 4A and 4B,repeating envelope patterns 34 and 134 result from the difference in thepitch of the visible pattern, as imaged or projected onto thephotosensor array, and the pitch of the photosensors. These envelopes34, 134 are reflected in the waveforms 30, 130. As is illustrated, theenvelope has a spatial frequency that is significantly lower than thefrequency of either the imaged (or projected) visible pattern or thephotosensor array, where the frequencies of the imaged or projectedvisible pattern and the photosensor array are equal to the reciprocal ofthe distance separating adjacent imaged or projected patterndemarcations or separating adjacent photosensor elements, respectively.In this regard, lateral motions of the object bearing the visiblepattern, made parallel to the direction of the repetition of therepeating patterns, produces a shift in the position of the lowerspatial frequency signal envelope. Even slight displacements of theobject bearing the visible pattern can be readily detected since theycause relatively large displacements of the envelope pattern which has alower spatial frequency than either the array geometry or that of theimaged or projected pattern.

FIGS. 4A and 4B illustrate the visible pattern as being linear and thephotosensor array as being two-dimensional. However, as will beappreciated by persons of ordinary skill in the art, other shapes andconfigurations may be implemented, consistent with the concepts andteachings of the present invention. In this regard, the preprintedpattern may be a concentric configuration of circles (see FIGS. 1A-1C),triangles (see FIGS. 3A-3B) or a variety of other configurations. Thelinear configurations illustrated may be preferred, as the processingrequired by the circuitry will generally be simpler than when a morecomplex configuration is implemented.

As will be appreciated by persons skilled in the art, the presentinvention may also be used to measure magnification changes using Moiréamplification. In this regard, with a lens that is not telecentric,movements of an object plane in and out of the intended field planecause changes in magnification that can be precisely measured by usingthe Moiré envelope signal to detect changes in the pitch of a regularpattern on the object surface relative to the pitch of pixel pattern(s)in photosensor arrays. A microscope is an example of an environmentwhere this feature of the invention may be utilized. For example, it maybe used in a microscope having a built-in photosensor array imager and acalibration pattern on a calibration object surface.

Another aspect of the present invention is to provide a system formeasuring the tilt of a device. In this regard, reference is made toFIGS. 5A and 5B which illustrate a tilt-measuring system constructed inaccordance with this other aspect of the present invention. In thesefigures, FIG. 5A illustrates the projection of an image from a firstfocal plane 160 onto the two photosensor arrays 156 and 158, and FIG. 5Billustrates the projection of an image from a second focal plane 162onto the same two photosensor arrays 156 and 158. In accordance withthis aspect, the system includes a telecentric lens 150 that, asmentioned above, includes a pair of lenses 151, 152 and an aperture stop153 that allows the object surface, having the visible pattern, todeviate from the mid-focal plane 161 of the lens 150, withoutsignificantly affecting the pitch of the image pattern at the array byan otherwise change in effective magnification, although not withoutblurring. The individual lenses 151, 152 and aperture stop 153 of thetelecentric lens 150 are aligned along a central axis. Object points onthe plane 161 will be in best focus at focal points on the image plane157, and blurred elsewhere. A first photosensor array 156 is disposedalongside the central axis 154 and angled with respect to the centralaxis. Similarly, a second photosensor array 158 is disposed alongsidethe central axis 154 and, like the first photosensor array 156, isangled with respect to the central axis 154. Indeed, the secondphotosensor array 158 is angled at an angle substantially opposite theangle of the first photosensor array 156. Further, the first and secondphotosensor arrays are disposed on a side of the telecentric lens 150opposite the object (not shown) and to opposite sides of the axis 154.Thus, object points on any parallel plane to planes 160, 161, or 162will be in best focus at some conjugate image points on the two arrays156 and 158, while object points at other distances from the planecontaining the axis 154 and orthogonal to the plane of the figure willbe blurred. Finally, a circuit 164 is provided in electricalcommunication with the first and second photosensor arrays 156, 158. Thecircuit 164 is configured to locally evaluate relative image contrastsfrom the electrical signals output from the first and second photosensorarrays 156, 158 to determine the tilt of the object.

As shown in the drawings, the first and second photosensor arrays 156and 158 are angled to achieve best focus of at least a portion of anobject surface disposed between a first image plane 160 and a secondimage plane 162. As illustrated, the upper portions of the photosensorarrays 156 and 158 correspond to the first focal plane 160, and thelower portions of the photosensor arrays 156 and 158 correspond to thesecond focal plane 162. That is, the image of an object that iscoincident with the first focal plane 160 will be projected by the lens150 to be in best focus on the upper portion of the photosensor arrays.Thus, as shown in FIG. 5A, points on the focal plane 160 but outwardfrom the axis 154 project through the lens 150 and focus best at pointscorresponding to the upper portions of the photosensor arrays.Conversely, points on the focal plane 162 and outward from the axis 154project through the lens 150 and focus best at points corresponding tothe lower portions of the photosensor arrays. If a portion of an objectis located between the focal planes 160 and 162 on plane 161, then thepoints on the object from these points will focus best on the arrays 156and 158 where these arrays intersect the conjugate plane 157, at pointsnearer the axis 154 than those shown. As will be appreciated, theseparation distance between the focal planes 160 and 162 will bedetermined by the size and angular disposition of the photosensor arrays156 and 158 and the depth of field of the lens 150. Thus, the distanceof separation between the focal planes 160 and 162 will increase as thesize (length) of the photosensor arrays 156 and 158 increases (for agiven angle), and as the angle to the horizontal of the photosensorarrays increases (for a given length).

A circuit 164 is disposed to receive and evaluate electrical signalsgenerated by the photosensor arrays 156 and 158. In this regard, thecircuit 164 will preferably include a processor (not shows) that may beconfigured to execute segments of code that control the operation of theprocessor to evaluate the signals received from the photosensor arrays156 and 158 to carryout the operation of the system, as generally setout herein.

As will be described in more detail in connection with FIGS. 6A, 6B, 6Cand 7, the crossed configuration of the photosensor arrays 156 and 158allows the system to determine whether an object surface is level (i.e.,parallel with the focal planes 160, 161, and 162), or tilted. As will beappreciated by those skilled in the art, there will be a variety ofapplications for the system of FIGS. 5A and 5B.

The system, however, will detect the surface of an object disposedbetween the first and second focal planes 160 and 162 by detecting alight pattern on the surface of the object. As will be appreciated bypersons skilled in the art, the pattern on the object may be a createdpattern that is preprinted on the object. Alternatively, it may be asurface pattern that is inherent in the material itself, such as fiberpatterns in plain paper. Alternatively, it could be either a natural orcreated pattern disposed on the object. In this regard, the pattern maybe that of a natural material pattern inherent to structure-relatedproperties of the material of the object. “Inherent structure-relatedproperties” are defined herein as properties of the original that areattributable to factors that are independent of forming image dataand/or of systematic registration data on the original. “Inherentstructural features” are defined herein as those features of an originalthat are characteristic of processes of forming the original and areindependent of forming image data and/or systematic registration data onthe original. Often, the inherent structural features are microscopic,e.g., between 1 and 100 μm, features of surface texture.

In one embodiment, the photosensor arrays may be one-dimensional arrays(e.g., either a column or a row of photosensor elements). In such anembodiment, the system will detect tilt in one dimension of the object(e.g., left-right tilt). In an alternative embodiment, the photosensorarrays may be two-dimensional arrays. In such an embodiment, the systemwill detect tilt in either (or both) of two dimensions. In this regard,FIGS. 6A and 6B illustrate the system using one-dimensional photosensorarrays and FIGS. 7A and 7B illustrate the system using two-dimensionalphotosensor arrays.

To better illustrate the operation of the system of FIGS. 5A and 5B,reference is now made to FIG. 6A, which illustrates the first and secondphotosensor arrays 156 and 158 along with a graphical depiction 166 and168 of the intensity of light sensed by the photosensor arrays 156 and158 respectively. For purposes of simplifying the illustration, assumethat each of the photosensor arrays 156 and 158 are one-dimensionalarrays, each having seventeen photosensor elements. Each photosensorelement generates an electrical signal that corresponds to the intensityof the light received by that photosensor element. A hypotheticalexample is present in blocks 166 and 168. Specifically, each of theseblocks has seventeen individual sub-blocks. The shading within thesesub-blocks is representative of the light incident upon that photosensorelement. Thus, the rightmost and leftmost sub-blocks of blocks 166 and168 contain no shading, representing regions of relatively low contrast.The middle sub-blocks have alternatingly dark and light sub-blocks,indicating higher contrast among those photosensor elements. Thus, theimage of the object is in good focus in the middle regions of arrays 166and 168. It will be appreciated that the example illustrated in thedrawings is an extremely simplified example that has been simplifiedmerely for purpose of illustration. In practice the photosensor elementswill be of an analog nature, having analog outputs, and therefore willsupport an infinite spectrum of varying light intensities and focus.Also, there will, in practice, be many more photosensor elements thanthe relatively few seventeen elements of FIGS. 6A and 6B. Theinterpretation from the illustrated positions of best contrast is thatthe object must be positioned coplanar with plane 161 (See FIGS. 5A and5B).

The particular magnitude of best focal contrast of the variousphotosensor elements is not significant for purposes of the invention,but only its magnitude relative to other positions of the arrays.

In contrast, and as illustrated in FIG. 6B, the regions of highestcontrast from the photosensor elements of the first and secondphotosensor arrays 156 and 158 (as represented by block 166 and 168) areshifted outward from center in opposite directions. The high-contrastpattern in photosensor array 156 (block 166) has moved to the rightwhile that of the photosensor array 158 (block 168) has moved to theleft, indicating that the surface of the object is level or parallel tothe focal planes 160, 161, and 162, but displaced closer to plane 160.The degree to which the photosensor elements differ will be evaluated bythe circuit 164 to determine the magnitude of tilt of the target object.

FIG. 6C illustrates a tilt of the object about an axis lying in plane161 and normal to the page. As shown, the contrasts reflected in blocks166 and 168 represent a tilt about an axis that puts the object morenearly parallel to array 156 and less parallel to array 158.

Reference is briefly made to FIG. 7 which is similar to the illustrationof FIG. 6A, except that the embodiment illustrated in FIG. 7 utilizestwo two-dimensional photosensor arrays 256 and 258. Blocks 266 and 268represent the contrast intensities detected by the photosensor elementsof the photosensor arrays 256 and 258, respectively. As illustrated, thecontrasting sub-blocks of the blocks 266 and 268 of FIG. 7 indicate thatthe object surface includes the line of intersection between plane 161and the plane of FIG. 7, but is tilted about that line as an axis ofrotation.

Although not shown in the drawings, a beam splitter may be insertedbetween the lens 150 and the arrays 156, 158 and the photosensor arrays156 and 158 repositioned so that they both view the same region of theobject surface. In the preferred embodiment, the arrays 156 and 158 arelaterally offset, so that they view a laterally offset region of theobject surface. Although such a configuration is generally acceptablefor purposes of detecting object tilt (particularly where the objectsurface is planar), it may be desired in some environments to have thearrays 156, 158 to view the same region of the object. Properconfiguration of a beam splitter between the lens 150 and the arrays156, 158 would achieve this goal.

As will be appreciated, the use of two-dimension photosensor arrays 256and 258 facilitates the detection of tilt in each of two differentdirections. Also, one skilled in the art will appreciate that the arrayscan be tilted in different ways than illustrated herein. Furthermore,prisms could be used to give the effect of tilting the arrays, whereinthe arrays can then be coplanar or even become portions of a largersingle array. For example, the arrays may be disposed in an untilted(horizontal) fashion and prisms placed between the arrays and the object(preferably adjacent the arrays). The prisms could be shaped orotherwise configured to redirect the light passing through them andthereby impart “effective” tilts to the arrays. For example, awedge-shaped prism could be placed in front of each of two arrays. Inthis way, the remainder of the system may be configured to operate asdescribed herein.

In yet another configuration, a single, circular photosensor array (andpossibly a circularly symmetric prism) may be used. Photosensor elementsmay be distributed across the entire array. The image contrast detectedby the various photosensor elements may be evaluated and correlated todetermine the tilt of an object surface. In another configuration athree dimensional array with conical coordinates, or a two dimensionalarray with circular coordinates and an inverse conical prism. Thesevariations of the invention, however, are all within the broad conceptsand teachings discussed herein, and need not be specifically describedfurther.

Reference is now made to FIG. 8, which illustrates yet anotherembodiment of the present invention. In accordance with this embodimentof the invention, the system includes two different lenses 372 and 374oriented in a similar, but slightly different direction. Moreparticularly, a first lens 372 having a relatively deep depth of fieldis disposed along a first optical axis 373. Similarly, a second lens 374also having a relatively deep depth of field is disposed along a secondoptical axis 375. The second optical axis 375 is slightly angled withrespect to the first optical axis 373 so that they are not parallel, butin such a way that the first and second lenses 372 and 374 have a sharedfield of view in which a target object 383, to be tracked, may belocated. In this regard, a first photosensor array 356 is disposedsubstantially orthogonal to the first optical axis 373 and opposite theshared field of view. Likewise, a second photosensor array 358 isdisposed substantially orthogonal to the second optical axis 375 andopposite the shared field of view. Finally, a circuit 364 is disposed inelectrical communication with the first and second photosensor arrays356 and 358, wherein the circuit 364 is configured to evaluateelectrical signals output from the first and second photosensor arrays356 and 358 to track the movement of an object 383 within the sharedfield of view.

Preferably, the lenses 372 and 374 are positioned to provide thephotosensor arrays 356 and 358 deep depths-of-fields ranging from a nearobject plane 376 to a far object plane 378. As illustrated, the opticalaxes 373 and 375 are angled with respect to one another, and intersectat point 383 in object field 377. As will be appreciated by personsskilled in the art, the system of FIG. 8 may be configured such that theimages received by the photosensor arrays 356 and 358 are nearlyidentical for an object positioned at point 383 in the object plane 377,except for the small difference caused by the difference in perspective.

If, however, such an object moves out of the object plane 377 to a newobject plane (at or between the planes 376 and 378), the respectiveimages received by the photosensor arrays 356 and 358 will shift withinthe planes of their image frames. The direction and magnitude of thisshift will be indicative of the displacement between the object planes.Such shifting is revealed by first observing that centers of the imageframes of the two photosensor arrays 356 and 358 will be coincident withthe respective images of the object point 383, which lies at theintersection of the two optical axes 373 and 375. When an object surfacemoves from the object plane containing this intersection point 383 toanother object plane, such as the near or far object planes 376 and 378,the points on the object surface that move to the centers of the arraysbecome two points: 386 and 382 on the near object plane 376, and 388 and384 on the far object plane 378. Measurement of the differences in imageshifting between the two arrays can be used to interpret thethree-dimensional movement of the object surface. In this regard,measurements of image shifts can be performed by, for example, aprocessing circuit 364, by correlation techniques between successiveimages or their sub-images, or by heuristic techniques known andunderstood by those skilled in the field of image motion tracking.

The foregoing description is not intended to be exhaustive or to limitthe invention to the precise forms disclosed. Obvious modifications orvariations are possible in light of the above teachings. In this regard,the embodiment or embodiments discussed were chosen and described toprovide the best illustration of the principles of the invention and itspractical application to thereby enable one of ordinary skill in the artto utilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. All suchmodifications and variations are within the scope of the invention asdetermined by the appended claims when interpreted in accordance withthe breadth to which they are fairly and legally entitled.

What is claimed is:
 1. A system for taking displacement measurements ofan object comprising: an object with a surface area presenting a visiblepattern, said pattern being projected as an image pattern onto aplurality of photo sensors, said image pattern comprising systematicchanges in radiant intensity; said plurality of photosensors uniformlyspaced apart in a configuration similar to the imaged pattern; and acircuit electrically connected to the plurality of photo sensors, thecircuit configured to evaluate an electrical signal generated by theplurality of photosensors to determine the displacement of the object.2. The system as defined in claim 1, further including a lens displacedto project the pattern of the object as an image pattern onto theplurality of photosensors.
 3. The system as defined in claim 1, whereinthe displacement of the object includes translational displacement. 4.The system as defined in claim 1, wherein the displacement of the objectincludes rotational displacement.
 5. The system as defined in claim 1,wherein the visible pattern is a series of parallel, linear marks thatare uniformly spaced apart.
 6. The system as defined in claim 1, whereinthe plurality of photosensors are disposed in an array.
 7. The system asdefined in claim 5, the uniform space between photosensors is slightlydifferent than the uniform space between the parallel, linear marks. 8.The system as defined in claim 1, wherein the circuit includes a centralprocessing unit and a memory.
 9. The system as defined in claim 2,wherein the lens is a telecentric lens.
 10. The system as defined inclaim 6, wherein the array is one-dimensional.
 11. The system as definedin claim 6, wherein the array is two-dimensional.
 12. The system asdefined in claim 1, wherein the plurality of photosensors is disposed ina circularly symmetric array.
 13. A system for taking displacementmeasurements of an object comprising: an object with a surface areapresenting a visible pattern; projecting means for projecting saidvisible pattern into an image pattern, the image pattern comprisingsystematic changes in radiant intensity; photosensor means for receivingthe image pattern; and circuit means for evaluating an electrical signalgenerated by the plurality of photosensors to determine the displacementof the object; wherein the photosensor means includes photosensorelements that are spatially configured in a pattern substantiallysimilar to the image pattern so that the circuit means can efficientlydetect even small displacements in the object by measuring a phase of anenvelope of the electrical signal.
 14. A system for taking displacementmeasurements of an object comprising. an object with a surface areapresenting a visible pattern; a telecentric lens spaced apart from theobject and configured to project the visible pattern as an imagepattern, the image pattern comprising systematic changes in radiantintensity; a plurality of photosensors disposed to receive the projectedimage pattern, the plurality of photosensors disposed in a configurationthat is sustantially similar to the image pattern, the configuration ofthe photosensors having only a slightly different pitch than the imagepattern; and a circuit electrically connected to the plurality ofphotosensors to receive an electrical signal generated by the pluralityof photosensors, the circuit being specifically configured to evaluate aphase of an envelope of the electrical signal, wherein the phase is usedto determine the displacement of the object.